Among the polyester fibers, those based on polyethylene terephthalate continue to be the most important, although fibers based on 1,4-dimethylenecyclohexane terephthalate have become commercially available. Developments in both homo- and copolyesters have continued and many modified versions of polyethylene terephthalate have recently appeared on the market. With the advent of new fibers, the search has continued for dyes which build up on the various types of polyester fabric proportionate to the amount of the dye applied, and which are characterized by good light and excellent sublimation properties.
In polyethylene terephthalate fibers, the diffusion of the dye into the fiber is effectively controlled by the mobility of the chain molecules in the disordered regions. However, the molecular shape and the size of the dye, the presence of polar groups and general steric considerations are also important in relation to the rate of diffusion.
We have discovered that certain water insoluble azo dyes diffuse well into the now available polyester fibers. These dyes can be applied by any of the standard methods and the resulting dyeings are characterized by good light fastness and outstanding sublimation properties.
Dyes bearing general structure similarity to those of the present invention were first disclosed by Joseph B. Dickey in U.S. Pat. No. 2,346,013 as colors for cellulose, wool and silk. Structurally similar dyes, described as colorants for polyester, cellulose acetate, polyamide and polyalkenes are described in Dutch Patent 138,695. None of the dyes described in the Dutch Patent can be made using the coupler of the dyes of the present invention.